Apparatus and Method for Forming Fibre Reinforced Composite Structures

ABSTRACT

A method and apparatus for forming a fibre reinforced composite structure comprising at least two components, a hard base tool presenting a first tool face, a second tooling element presenting a second tool face to locate the second component against the first component. Components are of a dry fibrous form and are to be injected with a liquid resin. A flexible bagging blanket forms with the tools a sealed enclosure into which the resin is injected to form the structure by resin transfer infusion. To maintain the relative dispositions of the components without the need for elaborate mandrel plates, control rods are inserted at least partially through the two components to act as fasteners and to maintain the relative disposition of the components during resin injection. After injection the assembly is cured and the control rods may remain in situ or be removed from the structure.

This invention concerns an apparatus for and method of forming a fibrereinforced composite structure having at least two components conjoined,the structure being formed by providing the components in the form ofreinforcing fibrous preforms, locating them face to face on or betweenhard base tool surfaces, injecting a liquid resin into the fibrouspreforms and curing the resin to form the composite structure.

The use of advanced composites, specifically carbon/epoxy materials forthe manufacture of, for example, airframe structures, has, in recentyears, been used significantly more commonly for both commercial andmilitary aircraft. The object has been to produce lightweight, corrosionand fatigue resistant structures. Specifically, weight reductions ofbetween 15% and 25% have been achieved due to improved strength andstiffness, and this has resulted in considerably reduced maintenance andinspection costs as a result of the improved fatigue resistantproperties achieved. The technique enables the production of smoothaerodynamic profiles while avoiding the high tooling costs incurred inproducing metallic components with complex three dimensional curvature.

Autoclave cured “pre-pregs”, ie, individual fibrous components which arepre-impregnated with resin and then assembled as required, have beendeveloped but traditionally are costly to manufacture and incur processdifficulties.

Consequently, laminating processes have been developed which involveinfusion of low viscosity structural resin into “dry” carbon fibrepreforms which are preformed layers of carbon fibres fixed inpredetermined orientations. Such processes are often referred to as‘resin transfer moulding’ and ‘resin transfer infusion’. Typicalstructures to be produced in this way are wing and stabiliser structuresfor aircraft, in which, for example, it may be required to produce awing skin with spaced, generally parallel stiffeners such that the skinand the stiffeners must be securely fastened together in such a way asto avoid undue stresses in the finished components, and wherein the needto utilise material-expensive and time consuming procedures with largenumbers of mandrels applied to the structure during production, isavoided.

High dimensional accuracy in the formation of such components isachievable with the resin transfer moulding process which uses hardmatched tooling to both faces of the tool. In this process one or moredry fibrous preforms are firstly compressed in a mould cavity formed byupper and lower matched hard tooling parts. Liquid resin is theninjected into the mould cavity under pressure with the intention offully impregnating the preforms. If two components are involved thenthese are formed together with accuracy owing to the matched toolingparts. The injection of liquid resin may be carried out with or withoutvacuum assistance.

Because of the hard tooling on both faces of the preforms, the resin isinjected edgewise into the fibrous preform and must therefore infusethrough the entire preform to achieve complete wet out. There arelimitations to the resin transfer moulding process. These include:

-   a) considerable costs incurred for the two-part hard tooling,    particularly for large components;-   b) the dimensional accuracy and relative positioning of the finished    components, which depends upon the compressibility of the fabric    preforms coupled with the tooled stiffness, the matching accuracy of    the tool and the injection pressure;-   c) the time taken to enable complete infusion of the preforms may be    outside the usable resin injection time;-   d) the forces imposed on the tooling edges can be high during tool    closing, leading to possible damage, thickness variation or movement    of the preforms;-   e) perfect matching of the upper and lower tools. (The sealing and    high vacuum integrity of the tool can be difficult to achieve    especially for complex and large components); and-   f) unless the coefficients of thermal expansion of the tool parts    and the preforms are closely matched the dimensional accuracy may be    compromised.

Since the alternative resin transfer infusion process described inspecification GB 2316036 uses a flexible, usually elastomeric, baggingblanket which cooperates with a single hard based tool to form thesealed enclosure, some of the aforementioned difficulties are overcome.In this case, the preform, with or without a further preform component,is laid up on the base tool and the flexible blanket is applied over thepreform, the whole assembly being placed in an autoclave. Vacuum isapplied to the area inside the bag to evacuate air from the preformwhile liquid resin is allowed to be drawn into the preform. The flexibleupper tooling provided by the blanket helps facilitate resin infusionacross the upper surface of the preform in contrast to the resintransfer moulding process which is largely edge infusion. Thus, largerand more complex components can be formed using the RTI process.

However, the use of flexible upper tooling makes it difficult to attainaccurate final thickness of the component, since the flexible blankettakes up a final position dependent upon various factors, including thequantity of resin required, the aerial weight of the fabric, the resinrheology, gel and cure characteristics, the vacuum level in the preformprior to injection, the resistance offered by the blanket and theexternal pressure applied, and the wet out efficiency of the fibrouspreform. This difficulty can be avoided largely by incorporatingthickness control plates and spacer rods.

In addition, where two fibrous preforms are superimposed beneath theblanket, it is necessary to ensure that the components remain in thecorrect disposition and do not suffer relative movement during resininfusion. Accurate positioning has been achieved by using a complex andexpensive matrix of removable mandrels to determine the relativepositions of the components prior to and during resin infusion.

It is an object of the present invention to avoid the use of suchmandrels by providing alternative means for ensuring location andrelative disposition of the preform components.

According to one aspect of the present invention there is providedapparatus for forming a fibre reinforced resin composite structurehaving at least two components, comprising a hard base tool whichpresents a tool face having a lay-up region for lay-up of a firstreinforcing fibre preform; a second tooling element which presents asecond tool face over the lay-up region to locate a second reinforcingfibre preform against the first reinforcing fibre preform; a flexiblebagging blanket for overlying the lay-up region and cooperating with thetool faces to form a sealed enclosure which encloses the first andsecond reinforcing fibre preforms; a liquid resin source; and a liquidresin inlet which communicates with the sealed enclosure and which isconnected to the liquid resin source for injection of liquid resin intothe sealed enclosure to form a liquid resin/reinforcing fibre preformsystem for liquid resin impregnation of the reinforcing fibre preforms;characterised by at least one rigid control rod inserted in a bore so asto pass at least partially through the first and second reinforcingfibre performs to maintain the relative disposition of said preformsduring the liquid resin injection.

A vacuum outlet may communicate with the sealed enclosure and which isadapted in use to be connected to vacuum generating means to create atleast a partial vacuum within the sealed enclosure.

The or each control rod may be permanently inserted within the performs.

The or each control rod may be removably inserted within the preforms.

Said at least two components may have co-planar surfaces disposedface-to-face.

The or each control rod may pass completely through a part of one of thepreforms and only partially through a part of the other.

The or each control rod may pass completely through respective parts ofboth preforms.

A spacer rod may pass through a part of one of the preforms and abut theadjacent face of the other.

Said at least one control rod may be at least partially threaded anddisposed so as to fasten the components together prior to resininjection.

At least one of the preforms may be pre-drilled to receive the or eachat least partially threaded control rod, the diameter of the or eachpre-drilling in at least one of the preforms being equivalent to theminimum diameter of the thread of the rod.

The or each threaded control rod may be threaded throughout its length.

The pre-drilling may be to a depth less than the length of the rod to beinserted therein, thus to leave a pre-drilled zone free for resinfilling.

The or each control rod, or at least one of a plurality thereof, may bemetallic.

The or each control rod, or at least one of a plurality thereof, may beformed as a reinforcing fibre preform.

According to a further aspect of the invention there is providedapparatus for forming a fibre reinforced resin composite structure asaforesaid, in combination with an autoclave adapted to contain andenclose an assembly comprising the hard base tool, the second toolingelement, the reinforcing fibre preforms and the flexible baggingblanket.

According to a still further aspect of the present invention there isprovided a method of forming a fibre reinforced resin compositestructure having at least two components, comprising the steps ofproviding a hard base tool which presents a tool face having a lay-upregion for lay-up of a first reinforcing fibre preform, providing asecond tooling element which presents a second tool face over the lay-upregion to locate a second reinforcing fibre preform against the firstfibre reinforcing preform, providing a flexible bagging blanketoverlying the lay-up region and cooperating with the tool faces to forma sealed enclosure enclosing the first and second reinforcing fibrepreforms, injecting a liquid resin into the sealed enclosure to form aliquid resin/reinforcing fibre preform system causing liquid resinimpregnation of the reinforcing fibre preforms; and allowing the resinto cure; characterised by the pre-insertion of at least one rigidcontrol rod in a bore so as to pass at least partially through the firstand second reinforcing fibre preforms to maintain the relativedisposition of said preforms during the liquid resin injection.

At least a partial vacuum may be created within the sealed enclosure.

The or each control rod may remain permanently inserted within thepreforms.

The or each control rod may be removed from the preforms after curing.

The components may have co-planar surfaces and be disposed face-to-facewithin the enclosure.

The method may include the step of enclosing the assembly comprising thehard based tool, the preforms and the flexible bagging blanket, withinan autoclave in which a predetermined pressure is established duringresin injection and subsequent thereto for curing.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates a conventional system for forming afibre reinforced composite structure such as an aircraft wing with sparsor cross-members;

FIG. 2 isometrically illustrates parts of the system of FIG. 1;

FIG. 3 is a part-sectional view of a part of a two-component compositestructure;

FIG. 4 is a similar view showing a different embodiment;

FIG. 5A is an enlarged part-sectional view showing a fastener for thetwo components;

FIGS. 5AA and 5AAA are enlarged part-sectional view showing alternativefasteners for the two components

FIGS. 5B to 5D are further enlarged views of a threaded portion of thefastener in situ;

FIG. 6 is a view showing a drilled aperture for receipt of the fastener;

FIG. 7 shows a different form of fastener;

FIG. 8 illustrates a typical application of a multi-component reinforcedresin composite structure made in accordance with the invention;

FIG. 9 shows a different form of fastener prior to formation of thestructure; and

FIG. 10 is a view similar to FIG. 9, post-formation.

Referring now to FIGS. 1 and 2, in a conventional system a firstreinforced fibre preform 10 to be used, for example, in the constructionof an aircraft wing skin, is placed on a hard base tool 11 configured torepresent the required curvature for the first preform 10. Stiffeners 12are then placed face to face upon the preform 10 and each stiffener 12consists of a flange 13 (seen clearly in FIG. 3) and an upstanding web14 substantially perpendicular to the flange 13. Each stiffener 12 isoverlaid with a two-part upper tool 15.

A duct 16 (see FIG. 3) is provided for the supply of liquid resin to thepreforms 10 and 12 whereby resin may be injected into the fibrousstructure of the two preforms so that they become fully impregnatedduring the formation process.

A flexible bagging blanket 17 is disposed beneath a support structure 18with bars 19 to cause the bagging blanket to form over and around thepreforms 10 and 12 during resin injection. Clamping devices 20 may beprovided to force the support structure 18 downwardly over the assemblyof parts.

Conventionally, a complex and thus costly rib and mandrel plate system21 has been interposed between the bagging blanket 17 and the uppersurfaces of the preforms. Such a rib and mandrel plate is illustrated inFIG. 2 and this requires to be secured to the hard base tool 11 at 22.

During the resin injection process although the flexible blanket 17locates, to some extent, the parts of the assembly in the correctdisposition, however the rib and mandrel plates 21 are required toensure accurate location at every position of the stiffeners 12, sincethe blanket 21, being flexible, may allow some movement during resininjection.

Several such rib and mandrel plates 21 are required across the assembly.

Referring now to FIG. 3 and in accordance with a first embodiment of theinvention, the rib and mandrel plates are replaced by fasteners orcontrol rods which, permanently or removably, pass at least partiallythrough the first and second reinforcing fibre preforms to maintain therelative disposition of said preforms during the liquid resin injection.

In this example a first control rod 23 passes through the flange 13 ofthe stiffener 12 and partially through the preform 10. The rod 23 may beof metal or of ceramic material, or of a composite material. In theexample shown, a further control rod 24 of a fibrous composite materialalso passes through the flange 13 and partially through the preform 10.In this case if the composite nature of the stiffener 12 is not fullycompacted the composite material rod 24 facilitates some small movementduring resin injection. A small cavity is left beneath the rod 23 whichwill fill with injected resin, or which can be occupied by a smallamount of soft glass or ceramic.

Where the preform 10 forms the outer skin of an aircraft wing, lightningstrike protection is afforded by the fact that the control rods 23, 24do not extend fully through the preform 10.

Referring now to FIG. 4, in a further embodiment a control rod 25extends fully through the preform 10 and through the flange 13 therebycontrolling the movement of the preform 10 and stiffener 12 combination.

Referring now to FIG. 5A, each of the control rods 23, 25 of FIGS. 3 and4 may be formed with a threaded portion 28 so that the rod acts as afastener which not only prevents relative sliding movement of thepreform 10 and the flange 13 but also securely fastens the two togetherin an axial direction of the rod.

Referring now to FIG. 5AA, each of the control rods 23, 25 of FIGS. 3and 4 may be formed as a completely threaded rod 23 a having a slot 23 bfor a screwdriver or Allen key to drive the rod into place. This form ofrod is appropriate for use in joints which are principally subject to ashear load. The entire threaded length of the rod supports whateverminor tensile loads may exist.

Referring now to FIG. 5AAA, the controls rod 23, 25 of FIGS. 3 and 4 maybe formed with a non-threaded central portion 23 c to provide increasedstrength to shear loads as compared with the rod 23 a of FIG. 5AA.

Where a threaded rod of this kind is used, an aperture is pre-drilledthrough the flange 13 and partly through the preform 10 to allow thethreaded rod to be inserted. The pre-drilling is such as to leave asmall gap 29 beyond the end of the rod for consolidation of resin duringinjection and the rod is installed under slight axial tension also tofacilitate consolidation. Gap 29 may be filled with insulation materialsuch as ceramic or glass fibres to improve electrical isolation forlightning strike protection purposes. It will be appreciated that therod is inserted with the components in a dry condition, ie, prior toresin injection, and may incorporate a release interface to facilitateremoval if required.

FIG. 5A includes a diagrammatic rectangular portion which is shownenlarged in FIG. 5B, and FIG. 5C shows, further enlarged, how the dryfibres are forced into the thread 28 of the rod so that after resininjection the fibres may become partially re-aligned, as can be seen inFIG. 5D.

FIG. 6 illustrates the pre-drilling of the assembly at a first diameterd1 to accommodate the threaded part of the rod and a second, largerdiameter to receive the unthreaded part. The reduced diameter d1 isequivalent to the inner diameter d2 of the threaded part. The flange 13is also countersunk at D in the case where a rod with a countersink headis used.

Referring now to FIG. 7 there is shown an embodiment in which thethreaded portion 30 of a control rod has a deeper, smoother thread tominimise the risk of de-lamination of the preform layers, and in thiscase the pre-drilling will be at a uniform diameter equivalent to theinner diameter of the threads. Such a thread is shown also in FIG. 5AAA,by way of example. The form of thread may be determined to provideadequate resistance to the loading expected in service.

Referring now to FIG. 8 there is illustrated a practical application ofa multi-component reinforced resin composite structure comprising thepreform 10 and stiffeners 12 with threaded control rods 28. This exampleshows a cross-stiffener 31 while the stiffeners 12 have supporting webs32 through which are inserted further rods 33, 34. The entire assemblyillustrated in FIG. 8 may be formed in a single injection or multipleinjection process.

Referring now to FIG. 9, there is illustrated a pair of preformed dryfibre control rods 35 in place of the rods 23, 24 of FIG. 3 and in thiscase the rods 35 are of reduced diameter when compared with thepre-drilled apertures 36 in which they are to be installed, but the rodsare of excess length when compared with the apertures 36.

Referring now to FIG. 10, it will be seen that as the upper tool 15bears against the fibrous rods 35 during resin injection, the rods arecompressed to fill the apertures 36 thus to achieve an interference fitof the rods within their respective apertures. Also, some controlledmovement of the parts of the assembly may be afforded by use of thesetwo compressible fibre rods. The arrangement illustrated in FIGS. 9 and10 will provide increased strength of the assembly both before and afterresin injection.

It is not intended to limit the invention to the examples described andillustrated herein. Many different configurations of fibrous componentsmaking up the reinforced resin composite structure may be chosen asrequired, while the relative disposition of the two or more componentsis ensured by the use of at least one control rod between each adjacentpair of components which, during resin injection, ensures accuratelocation and relative disposition of the respective components. The oreach rod is inserted between the components in a dry state, ie, prior toresin injection, and so controls the positioning of the components asthe resin is infused into the fibrous structure of the components. Oncethe structure is completed and cured it is necessary only to remove itfrom the hard tooling whilst the need for complex and expensive rib andmandrel plates has been avoided.

The control rods, particularly those being at least partially threaded,may, if required, be pre-treated with a release agent so that they maybe removed after formation of the structure. This is particularly usefulwhere the structure is to be subsequently fastened to another structureor where a special kind of fastener is to be attached to the structureto receive, for example, cabling or the like.

1. Apparatus for forming a fibre reinforced composite structure havingat least two components comprising: a hard base tool which presents atool face having a lay-up region for lay-up of a first reinforcing fibrepreform; a second tooling element which presents a second tool face overthe lay-up region to locate a second reinforcing fibre preform againstthe first reinforcing fibre preform; a flexible bagging blanket foroverlying the lay-up region and cooperating with the tool faces to forma sealed enclosure which encloses the first and second reinforcing fibrepreforms; a liquid resin source; a liquid resin inlet which communicateswith the sealed enclosure and which is connected to the liquid resinsource for injection of liquid resin into the sealed enclosure to form aliquid resin/reinforcing fibre preform system for liquid resinimpregnation of the reinforcing fibre preforms; and at least one rigidcontrol rod inserted in a bore so as to pass at least partially throughthe first and second reinforcing fibre performs to maintain the relativedisposition of said preforms during the liquid resin injection.
 2. Theapparatus according to claim 1, wherein a vacuum outlet communicateswith the sealed enclosure and is adapted in use to be connected tovacuum generating means to create at least a partial vacuum within thesealed enclosure.
 3. The apparatus according to claim 1, wherein the atleast one rigid control rod is permanently inserted within the preforms.4. The apparatus according to claim 1, wherein the at least one rigidcontrol rod is removably inserted within the preforms.
 5. The apparatusaccording to claim 1, wherein the at least two components have co-planarsurfaces disposed face to face.
 6. The apparatus according to claim 1wherein the at least one rigid control rod passes completely through apart of one of the preforms and only partially through a part of theother.
 7. The apparatus according to claim 1, wherein the at least onerigid control rod passes completely through respective parts of bothpreforms.
 8. The apparatus according to claim 1, further comprising: aspacer rod passing through a part of one of the preforms and abuttingthe adjacent face of the other.
 9. The apparatus according to claim 1,wherein the at least one rigid control rod is at least partiallythreaded and disposed so as to fasten the components together prior toresin injection.
 10. The apparatus according to claim 1, wherein the atleast one rigid control rod is completely threaded from end to end andhas a slot for a screwdriver or Allen key to drive the rod into place.11. The apparatus according to claim 9, wherein the at least one rigidcontrol rod includes a non-threaded portion to provide increasedstrength to shear loads in the region of an interface between the firstand second reinforcing fibre preforms.
 12. The apparatus according toclaim 9, wherein at least one of the preforms is pre-drilled to receivethe at least one rigid control rod, the diameter of the pre-drillingbeing equivalent to the minimum diameter of the thread of the at leastone rigid control rod.
 13. The apparatus according to claim 12, whereinthe pre-drilling is to a depth greater than the length of the at leastone rigid control rod, thus leaving a pre-drilled zone free for resinfilling.
 14. The apparatus according to claim 13, wherein thepre-drilled zone is filled with an insulation material to improveelectrical isolation for lightning strike protection purposes.
 15. Theapparatus according to claim 1, wherein the at least one rigid controlrod is metallic.
 16. The apparatus according to claim 1, wherein the atleast one rigid control rod is a reinforcing fibre preform.
 17. Theapparatus according to claim 1 in combination with an autoclave adaptedto contain and enclose an assembly comprising the hard base tool, thesecond tooling element, the reinforcing fibre preforms and the flexiblebagging blanket.
 18. A method of forming a fibre reinforced resincomposite structure having at least two components, comprising:providing a hard base tool which presents a tool face having a lay-upregion for lay-up of a first reinforcing fibre preform, providing asecond tooling element which presents a second tool face over the layupregion to locate a second reinforcing fibre preform against the firstfibre reinforcing preform, providing a flexible bagging blanketoverlying the lay-up region and cooperating with the tool faces to forma sealed enclosure enclosing the first and second reinforcing fibrepreforms, injecting a liquid resin into the sealed enclosure to form aliquid resin/reinforcing fibre preform system causing liquid resinimpregnation of the reinforcing fibre performs, allowing the resin tocure, and pre-inserting at least one rigid control rod in a bore so asto pass at least partially through the first and second reinforcingfibre preforms to maintain the relative disposition of the preformsduring the liquid resin injection.
 19. The method according to claim 18,wherein the at least one rigid control rod is at least partly threaded,and an aperture is pre-drilled through one of the at least twocomponents and partly through the other to receive the at least onerigid control rod, the pre-drilling being such as to leave a gap beyondthe end of the at least one rigid control rod for consolidation of resinduring injection, the at least one rigid control rod being installedunder slight axial tension.
 20. The method according to claim 18,wherein at least a partial vacuum is created within the sealedenclosure.
 21. The method according to claim 18, wherein the at leastone rigid control rod remains permanently inserted within the preform.22. The method according to claim 18, wherein the at least one rigidcontrol rod is removed from the preform after curing.
 23. The methodaccording to claim 18, wherein the at least two components haveco-planar surfaces and are disposed face to face within the sealedenclosure.
 24. The method according to claim 18, further comprising:enclosing the assembly comprising the hard base tool, the preforms andthe flexible bagging blanket, within an autoclave in which apredetermined pressure is established during resin injection andsubsequent thereto for curing.